Preparation

ACTIVE: 5 hours

Whisk flour, cocoa, baking soda, and 1/2 teaspoon (generous) salt in medium bowl. Stir butter and next 5 ingredients in another medium bowl until smooth. Stir in flour mixture, then nuts. Cover and chill until firm enough to scoop, at least 4 hours. DO AHEAD Can be made 1 day ahead. Keep chilled.

Position 1 rack in top third and 1 rack in bottom third of oven; preheat to 325°F. Line 3 large baking sheets with parchment. Measure 2 level tablespoonfuls dough; roll between palms into ball. Place on prepared sheet. Using fingers, spread out dough to 3-inch-diameter disk. Repeat with remaining dough, spacing 5 inches apart.

Bake cookies 8 minutes; reverse sheets. Bake cookies until flat and beginning to darken around edges, about 10 minutes. Transfer cookies on parchment to rack (cookies will crisp as they cool). DO AHEAD Can be made 4 days ahead. Store airtight at room temperature.